This disclosure relates to integrated circuit memories wherein distinct binary logic values are represented by changeable high electrical resistance and low electrical resistance states of magneto-resistive memory elements contained in bit cells. During memory read operations, the different resistance states are distinguished using sense amplifiers that are switchably coupled to addressed bit cells. A sense amplifier compares a resistance-related parameter of an addressed bit cell, such as current amplitude at a given bias voltage, versus a related parameter in a reference circuit comprising reference magneto-resistive elements that produce a threshold level for comparison with the bit cell.
This disclosure provides for reducing the effects of differences in resistance among reference elements using certain averaging techniques, and also offsets the variation in the resistance of the conductors of different lengths, that switchably couple addressed bit cells to a sense amplifier. Conductors of a length comparable to the conductors addressing a bit cell are inserted into the reference circuit when the bit cell is read so as to offset resistance variation with differences in the length of addressing conductors.
A spin-transfer torque magnetoresistive random access memory (STT-MRAM) stores binary data values in the resistance states of magnetic tunnel junction (MTJ) memory elements. A magnetic tunnel junction or MTJ has superimposed magnetic layers separated by a nonmagnetic film or barrier layer, such as magnesium oxide. The two magnetic layers have magnetic fields that can be aligned in the same orientation (parallel to one another) or in directly opposite orientations (anti-parallel). These relative magnetic field orientations are two distinct states at which the MTJ will remain stable without application of electrical power.
In the state of parallel field orientations, the MTJ has a relatively lower serial resistance RL through the magnetic and barrier layers. In the antiparallel field orientation state, the MTJ has a relatively higher serial resistance RH. During write operations, the MTJ can be switched from the high resistance state (with anti-parallel magnetic fields) to the low resistance state (with parallel magnetic fields), or vice versa, using write currents of sufficient amplitude and with respectively opposite current polarities. During read operations, a read bias voltage (or current) is applied across an addressed bit cell MTJ element and the current (or voltage) resulting under Ohm's Law V=IR is compared to a threshold characteristic of a resistance between RL and RH. The result of the comparison is deemed the logic level of the bit cell.
During a read operation, the resistance of an MTJ element can be expressed as a current or voltage. Various techniques can establish threshold levels for comparison. Various reference circuit configurations can distinguish between high resistance and low resistance states by comparison with the threshold. As one example, a voltage that varies with resistance can be generated by passing a read current of given amplitude through the MTJ element. The voltage is compared against a reference voltage to distinguish high and low resistance states. It is also possible to apply a given voltage and compare the resistance-dependent resulting current to a current comparison reference. The relatively higher and lower resistance states of one or more elements represent logical ‘zero’ or ‘one’ binary values of a bit cell.
The difference in resistance between RH and RL for given MTJ can be substantial, e.g., a difference in resistance of 100% or 200%. But the resistance values of the MTJ elements in their high and low resistance states (RH and RL), and the difference between the resistances in the states of high and low resistance (RH minus RL) vary among MTJ elements in a given integrated circuit memory. The values of resistance and the difference between RH and RL also vary from one integrated circuit to another.
Some of the variation in resistance, particularly from one integrated circuit to another, is due to normal manufacturing variations. Small differences in the thickness of the magnesium oxide barrier layer can produce substantial differences in resistance. The variations in resistance and in the span between RH and RL present challenges in the selection of a threshold value to use for comparison when distinguishing whether an MTJ being read is in its high or low resistance state. One might assume nominal values of RH and RL, and set a comparison threshold halfway between the nominal RH and RL, but that comparison threshold will not be the optimal value for all memory circuits or for all addressable MTJ bit cells on a given circuit, because the actual RH and RL resistances of the MTJ elements is distributed over a population.
An optimal threshold is well spaced from the values observed in most or all of the bit cells in their high and low resistance states, so that variations such as offset in the comparison circuit inputs, and variations in the absolute or differential resistances of the bit cell RH and RL values, do not cause read failures (namely inability to distinguish the two resistance states of an MTJ element of a bit cell accurately, repeatedly and dependably).
What is needed are practical ways to arrive at the most appropriate comparison threshold to be used in the read-sense amplifiers of individual integrated circuits, given the variation in RH and RL from one integrated circuit to another. One useful can be to include exemplary reference MTJ elements in a reference circuit coupled to the read-sense amplifiers to define the comparison threshold. One or more reference MTJ elements can be maintained in respective RH and RL states or switched between RH and RL states. The reference circuit produces from the reference MTJ element(s) a threshold to be used discriminating the resistance states of the bit cells. The threshold falls between the RH and RL levels of the reference MTJ elements, ideally halfway between RH and RL. If manufacturing variations cause the RH and RL resistances of the bit cells on a circuit to trend high or low, then those manufacturing variations should also cause the resistances of the reference bit cells to trend in the same direction.
Using reference MTJ elements to define the basis of comparison when reading the bit cell MTJ elements is helpful but not a complete solution. There remains a variation in the resistance values of the MTJ elements on an integrated circuit. There are respective distributions of RH and RL in the populations of reference MTJ elements and also in the populations of bit cell MTJ elements, both on a given integrated circuit and among a population of integrated circuits.
Voltage, current and resistance parameters are related by Ohm's Law. The comparison circuit and the parameters that are used as the basis for comparison can be any parameters that vary with the resistances of MTJ elements (in the bit cells and/or in the reference MTJ elements) in their high and low resistance states. The parameters that are actually compared to distinguish resistance states may be voltages applied to a voltage comparator or current to a current comparator, etc., wherein the threshold defining comparator varies as a function of MTJ resistance. One typically does not measure or process a resistance value in a circuit. The measured or processed value is a voltage or current that varies due to changes in resistance.
In addition to the variations in RH and RL on an integrated circuit and between integrated circuits, the typical resistance levels RH and RL, and/or the differences between the high and low resistance levels RH-RL, are not substantial compared to the resistances of conductive paths found in the integrated circuit, including the paths over which an addressed bit cell is coupled to the memory sense amplifier for purposes of comparison. The conductive paths, for example, may comprise polycrystalline silicon (or “polysilicon”) of very limited width and thickness. Manufacturing variations affect the resistances that are coupled along such conductive paths in series with an addressed bit cell MTJ element whose resistance state is to be read. And the length of the conductive paths varies among the bit cell MTJ elements on an integrated circuit because some of the bit cell MTJ elements are in word lines that are near to their respective sense amplifiers (usually one sense amplifier being provided per bit position), and some are farther away. The polysilicon or other conductors contribute additional resistance variation to the RH and RL variation of the bit cell elements as a function of their distances from the sense amplifier.
If all sources of resistance variability other than the change of resistance state between RH and RL could be minimized, the ability of sense amplifiers to accurately discriminate for high and low resistance states would be improved, even assuming that sense amplifiers may have an offset at their inputs. The effectiveness of threshold comparison by the sense amplifiers is degraded by the variability of RH and RL resistances in bit cell MTJ elements and also in reference MTJ elements. That variability is aggravated by the additional variability in the resistance of conductors that switchably couple the sense amplifier at a bit position in a word to the corresponding bit cell MTJ in an addressed memory word at a variable distance from the sense amplifier. It would be advantageous to reduce or eliminate resistance variability, other than the change of resistance state, if it is possible to do so.